1. Field of the Invention
The present invention relates to a magnetic disk drive, more specifically to a magnetic disk drive such as a flexible magnetic disk drive and a rigid magnetic disk drive which acquires information for positioning a magnetic head from a reproduced signal of a servo pattern recorded on a magnetic disk.
2. Description of the Related Art
The most general servo pattern used for positioning a magnetic head of a magnetic disk drive comprises servo bits which are arranged in a staggered layout along the centerline, and a method for positioning (tracking) a magnetic head is known, in which difference between each of the servo bits amplitude is generated to acquire the information on the position in the track-width direction, as described in Japanese Patent Publication No. 47-32012.
FIGS. 2(a) and 2(b) are diagrams illustrating the outline of the conventional xe2x80x9camplitude-detecting servoxe2x80x9d system. FIG. 2(a) is a diagram showing the relationship between tracks and a servo pattern, and FIG. 2(b) is a diagram showing an example of reproduced signals obtained from a magnetic head moving on the servo pattern. The case of positioning a magnetic head 10 having a track width of Twr on the track #N is considered. As shown in FIG. 2(a), when the magnetic head 10 moves in the direction x in the drawing and passes on patterns P and A to D, the reproduced signals as shown in FIG. 2(b) are obtained. The white and black portions of each of the patterns P and A to D show that the direction of magnetization on the portions of one color of the servo pattern recorded on a magnetic recording medium is opposite to the direction of magnetization on the portions of the other color. That is, in the case of longitudinal recording, the directions of magnetization on the white and black portions are vectors pointing to the opposite directions to each other which have track-direction (x-direction) components in the surface of a medium, while in the case of perpendicular recording, the directions of magnetization on the white and black portions are vectors pointing to the opposite directions to each other which have components perpendicular to the surface of the medium. The pattern of FIG. 2(a) is schematic and actually agrees with the signal period of FIG. 2(b).
When a difference between the amplitude SA of the reproduced signal of the pattern A and the amplitude SB of the reproduced signal of the pattern B, SAxe2x88x92SB, is calculated and the magnetic head 10 is caused to move in the track-width direction y, the calculation results in N-POS signal as shown on the right side of FIG. 2(a). Similarly, when a difference between the amplitude SC of the reproduced signal of the pattern C and the amplitude SD of the reproduced signal of the pattern D, SCxe2x88x92SD, is calculated and the magnetic head 10 is caused to move in the track-width direction y, the calculation results in Q-POS signal as shown on the right side of FIG. 2(a). By using the above calculated N-POS and Q-POS signals of desired portions as positional signals, the current position of the magnetic head 10 can be known.
Meanwhile, as a servo system different from the above servo system, the so-called xe2x80x9cphase-detecting servoxe2x80x9d system is disclosed, for example, in Japanese Patent Laid-Open No. 60-10472. FIGS. 3(a) and 3(b) are diagrams illustrating the outline of the conventional xe2x80x9cphase-detecting servoxe2x80x9d system. The case of positioning the magnetic head 10 having a track width of Twr on the track #N is considered. When the magnetic head 10 moves in the direction x and passes on patterns P and A to C shown in FIG. 3(a), the reproduced signals as shown in FIG. 3(b) are obtained, for example. The representation of the white and black portions of each of the patterns P and A to C is the same as that of FIG. 2. The patterns are at azimuth angles with respect to the magnetic head 10. Since these angles are too small to make the degradation (azimuth loss) in the reproduced signal a problem, the shapes of the reproduced signals of the patterns are not so different from those of FIG. 2(b). However, the phases of the patterns A, B and C relative to the pattern P vary depending on the position of the magnetic head 10 in the track-width direction y. The phases of the patterns A, B and C on the track #N are defined as PA, PB and PC, respectively. The pattern of FIG. 3(a) is schematic and actually agrees with the signal period of FIG. 3(b).
When the phase shifts PBxe2x88x92PA and PCxe2x88x92PB are calculated, an example of the results of the calculations is shown on the right side of FIG. 3(a). By using the above calculated PBxe2x88x92PA and PCxe2x88x92PB signals appropriately as positional signals, the current position of the magnetic head 10 can be known. As a method for obtaining the phases PA, PB and PC from the reproduced signals of FIG. 3(b), a method which is disclosed in Japanese Patent Laid-Open No. 6-231552 can be employed, for example.
Further, an example of an amplitude pattern in combination with a method comprising detecting a phase change by distorting the waveform is disclosed in Japanese Patent Laid-Open No. 9-251736. This pattern records a pattern which includes not only the characteristic properties of the conventional amplitude pattern but also the change-with-time property that a portion of a waveform changes while another portion does not. Because of this pattern, the patterns C and D of FIG. 2(a) can be omitted.
Along with an increase in the track density of a magnetic disk drive, a variety of technical factors or factors associated with the production of the magnetic disk drive come to the surface as the factors inhibiting the increase in the track density. Of such factors, geometrical factors ascribed to the track width of a magnetic head and the shape of a servo pattern appears in the form of non-linearity in a positional signal. This includes the effect of a difference between a geometrical track width and an effective track width in a recording pattern in the track-width direction. Further, as for noise in a recording medium or reproduction circuit system, it can be gradually recognized as a relative reduction in signal-to-noise ratio (S/N) relative to the reproduced signal. Further, as for disturbance, the track density can reach its limit when the error for track following control exceeds a margin.
Further, as for the prior art in which the method for detecting a change in a portion of a waveform with time by distorting the waveform is employed together with the amplitude pattern, it has such a defect that as a result of distorting the waveform, other harmonic content is produced other than the harmonic content in the fundamental wave or the waveform before distortion, thereby decoding noise increases.
The present invention has been invented in view of the above points, and the object of the present invention is to provide a method and a device that exhibit better performance against the above factors inhibiting the increase in the track density than that of the current system.
To solve the above problems, in the present invention, the information on the servo pattern or on the position for positioning a magnetic head is multiplexed. That is, the improvement of positioning accuracy is intended by acquiring both the information on the amplitude and the information on the phase from the servo pattern simultaneously and using them effectively while having them complement each other.
It has been recognized that the non-linearity in the positional signal is caused by the mismatch between the width of the servo pattern and the track width of a reproducing head, particularly in the amplitude-detecting system. However, the positions where the non-linearity is liable to occur are limited to some local portions. Thus, by acquiring the information on the phase which is not susceptible to non-linearity together with the information on the amplitude and using the two types of information while having them complement each other, the problem of the non-linearity in the positional signal can be avoided.
As for noise in the positional signal, by using the information on the amplitude and the information on the phase in combination, the amount of information can be further increased and the S/N can be greatly improved.
As for disturbance, by acquiring the information on the velocity of the head in the track-width direction from the information on the phase while detecting the information on the position, a recording process can be terminated at an early stage, for example, when the disturbance is severe.
As means for acquiring the information on the amplitude from the reproduced waveform of the servo pattern, there is available a method in which the above signal is subjected to full-wave rectification, followed by an integral operation. The thus obtained result (amplitude value) reflects the amplitude value of the reproduced signal of the servo pattern. As another means, these is available a method in which the information on the reproduced waveform of the servo pattern is collected discretely and the information is expressed by means of the Fourier polynomial expression. This method will be described in detail hereinafter. As means for acquiring the information on the phase from the reproduced waveform of the servo pattern, there is used a method in which the information on the reproduced waveform of the servo pattern is collected discretely and the information is expressed by means of the Fourier polynomial expression, as has just been described above. The method will be described in detail hereinafter.
The above reproduced waveform of the servo pattern is a repetition of a predetermined waveform over several cycles. The number of waveforms to be sampled discretely per cycle (number of oversampling) is defined as N, and the number of the above repetition of a positional signal waveform P(n) is defined as L. n is the number of the waveform-sampling point. The P(n) can be expressed as the following [EQUATION 1] by the Fourier polynomial expression. It is necessary to determine the frequency of sampling (discrete sampling) according to the frequency of the waveform to be sampled. In other words, a servo signal decoder circuit originally has the information on the frequency of the waveform as the sampling frequency.                               P          ⁡                      (            n            )                          =                              A            0                    +                      [                          xe2x80x83                        ⁢                                                                                                      xe2x80x83                                        ∑                                                                              N                      2                                        -                    1                                                                    m                  =                  1                                            ⁢                              xe2x80x83                            ⁢                              {                                                                            A                      m                                        ·                                          xe2x80x83                                        ⁢                                          cos                      (                                              xe2x80x83                                            ⁢                                                                        2                          ·                          n                          ·                          π                          ·                          m                                                N                                            )                                                        +                                      xe2x80x83                                    ⁢                                                            B                      m                                        ·                                          xe2x80x83                                        ⁢                                          sin                      (                                              xe2x80x83                                            ⁢                                                                        2                          ·                          n                          ·                          π                          ·                          m                                                N                                            )                                                                      ⁢                                  xe2x80x83                                }                                      ⁢                          xe2x80x83                        ]                                              [EQUATION  1]            
wherein A0, Am and Bm are discrete Fourier coefficients and m is the order of discrete Fourier transformation. The above Fourier polynomial is uniquely determined once the discrete Fourier coefficients are determined.
The Fourier coefficients, when the sampling data on the reproduced waveform of the pattern is defined as f(n), are determined by the following [EQUATION 2], [EQUATION 3] and [EQUATION 4].                               A          0                =                              1                          L              ·              N                                ⁢                                                    ∑                                                      L                    ·                    N                                    -                  1                                                            n                =                0                                      ⁢                          f              ⁡                              (                n                )                                                                        [EQUATION  2]                                          A          m                =                              2                          L              ·              N                                ⁢                                                    ∑                                                      L                    ·                    N                                    -                  1                                                            n                =                0                                      ⁢                                          f                ⁡                                  (                  n                  )                                            ·                              cos                ⁡                                  (                                                            2                      ·                      n                      ·                      π                      ·                      m                                        N                                    )                                                                                        [EQUATION  3]                                          B          m                =                              2                          L              ·              N                                ⁢                                                    ∑                                                      L                    ·                    N                                    -                  1                                                            n                =                0                                      ⁢                                          f                ⁡                                  (                  n                  )                                            ·                              sin                ⁡                                  (                                                            2                      ·                      n                      ·                      π                      ·                      m                                        N                                    )                                                                                        [EQUATION  4]            
In the most simple case, the desired result can be obtained by determining the Fourier coefficients A1 and B1 when the order is 1 (m is 1). In the case of amplitude detection, the amplitude value is determined by the following [EQUATION 5], while in the case of phase detection, the phase shift from the base pattern is determined by the following [EQUATION 6].                                                         (                              A                1                            )                        2                    +                                    (                              B                1                            )                        2                                              [EQUATION  5]                                arctan        ⁡                  (                                    B              1                                      A              1                                )                                    [EQUATION  6]            
A servo signal decoder circuit can determine the amplitude of a partial signal by integrating the partial signal and its phase from the phase of the above sinusoidal function. Alternatively, the servo signal decoder circuit can determine the amplitude and phase of the partial signal from the amplitude and phase of the above sinusoidal function. The servo signal decoder circuit preferably decodes the positional signal of the magnetic head by weighting the information obtained from the amplitude and the information obtained from the phase differently.
By using the above processes, the information on the amplitude and the information on the phase can be obtained simultaneously from the decoded waveform. As compared with the conventional method in which only the partial information of a waveform is obtained, the amount of information on decoding increases, and the number of opportunities to use it effectively also increases. Consequently, according to the process of the present invention for multiplexing the information on the servo pattern or on the position, a variety of problems ascribed to the above increase in the track density can be solved.
That is, the magnetic disk drive according to the present invention is a magnetic disk drive comprising a magnetic recording medium having a servo pattern, a magnetic head that writes information on and reads the information from the magnetic recording medium, and a servo signal decoder circuit that decodes the positional signal of the magnetic head from the reproduced signal of the servo pattern, in which the reproduced signal of the servo pattern comprises a plurality of partial signals whose amplitudes and phases change simultaneously according to the position in the track-width direction of the magnetic disk, and the servo signal decoder circuit decodes the positional signal of the magnetic head by determining a sinusoidal function, which roughly agrees with each of the partial signals, based on the information on the frequency of the reproduced signal which has been stored therein in advance.
The servo signal decoder circuit can determine the amplitude of a partial signal by integrating the partial signal and its phase from the phase of the above sinusoidal function.
Alternatively, the servo signal decoder circuit can determine the amplitude and phase of the partial signal from the amplitude and phase of the above sinusoidal function. The servo signal decoder circuit preferably decodes the positional signal of the magnetic head by weighting the information obtained from the amplitude and the information obtained from the phase differently.
Further, the velocity vector of the magnetic head can be obtained by using the positional signals of the magnetic head which are at two distinct positions away from each other at a given distance in the track direction.
Once the velocity vector in the head-moving direction is found, it becomes possible to estimate whether the head is on a given track, at the data position before the next servo sector. By using this information, when jumping over tracks is expected by the shock or vibration caused by external sources during a writing process, the destruction of the data on adjacent tracks can be prevented beforehand by preventing the writing process.
Further, the magnetic disk drive according to the present invention is a magnetic disk drive comprising a magnetic recording medium having a servo pattern, a magnetic head that writes information on and reads the information from the magnetic recording medium, and a servo signal decoder circuit that decodes the positional signal of the magnetic head from the reproduced signal of the servo pattern, in which the servo pattern includes a plurality of patterns arranged on both sides of the center lines of tracks, the patterns on one side of the center line being shifted in the track direction from the patterns on the other side along the center line, and the plurality of patterns, each including two different types of phase patterns arranged in the track-width direction. This servo pattern can be easily created by the current servo track writer. The pattern-writing time is also the same as that of the conventional servo pattern.
Further, the magnetic disk drive according to the present invention is a magnetic disk drive comprising a magnetic recording medium having a servo pattern, a magnetic head that writes information on and reads the information from the magnetic recording medium, and a servo signal decoder circuit that decodes the positional signal of the magnetic head from the reproduced signal of the servo pattern, in which the servo pattern includes a plurality of patterns arranged on both sides of the center lines of tracks, the patterns on one side of the center line being shifted in the track direction from the patterns on the other side along the center line, and the plurality of patterns, each including N (N is a positive number of 3 or more) different types of phase patterns arranged in the track-width direction. This servo pattern can be easily created by the current servo track writer.
Further, the magnetic disk drive according to the present invention is a magnetic disk drive comprising a magnetic recording medium having a servo pattern, a magnetic head that writes information on and reads the information from the magnetic recording medium, and a servo signal decoder circuit that decodes the positional signal of the magnetic head from the reproduced signal of the servo pattern, in which the servo pattern includes a plurality of patterns arranged on both sides of the center lines of tracks, the patterns on one side of the center line being shifted in the track direction from the patterns on the other side along the center line, and the plurality of patterns, each being a pattern whose phase in the track direction changes continuously in the track-width direction.
Further, the magnetic disk drive according to the present invention is a magnetic disk drive comprising a magnetic recording medium having a servo pattern and divided into a plurality of zones in the track-width direction, a magnetic head that writes information on and reads the information from the magnetic recording medium, and a servo signal decoder circuit that decodes the positional signal of the magnetic head from the reproduced signal of the servo pattern, in which the reproduced signal of the servo pattern includes a plurality of partial signals whose amplitudes and phases change simultaneously according to the position in the track-width direction of the magnetic head, the frequency of the reproduced signal of the servo pattern varies from one zone to another, and the servo signal decoder circuit decodes the positional signal of the magnetic head by determining a sinusoidal function, which roughly agrees with each of the partial signals, based on the information on the frequency of the reproduced signal of the servo pattern which has been read out of each zone. The frequency of the reproduced signal of the servo pattern may vary not from one zone to another but from one track to another.